1. Field of the Invention
This invention relates to a printing apparatus, and more particularly, to a printing apparatus for printing on print paper using a plurality of line printing heads.
2. Description of the Prior Art
Conventionally, a printing apparatus of the type which prints information received from an external or built-in information source is normally designed to print on print paper of a relatively small size. In such a printing apparatus, normally a single printing head is provided.
Printing apparatus for printing on print paper of a large size such as the A-0 or A-1 size of Japanese Industrial Standards are also known. One such printing apparatus is disclosed, for example, in U.S. Pat. No. 4,660,052. In the printing apparatus disclosed, up to four thermal heads, each having heating resistors arranged in a row thereon are arranged in two rows such that the heating resistors thereon are arranged in a pair of parallel rows, and a pair of platen rollers are arranged in a parallel relationship to each other and in an opposing relationship to the pair of parallel rows of the heating resistors. Each of the thermal heads has a pair of marginal portions contiguous to the opposite ends of a row of the heating resistors on the head and hence has a greater dimension in the direction of the row of the heating resistors than the length of the row. Accordingly, in order to attain a print of a complete printing line with the four thermal heads, such a zigzag arrangement of the thermal heads as described above is effective.
The thermal heads disclosed in U.S. Pat. No. 4,660,052 are of the type wherein the heating resistors are provided at a substantially central location on the thermal head in the feeding direction of paper perpendicular to the rows of the heating resistors. Accordingly, the thermal heads must be arranged such that the two rows of the heating resistors thereon are spaced from each other by a distance greater than the length of each of the thermal heads in the feeding direction of paper, and therefore, the heating resistors in the two rows are spaced by a comparatively great distance from each other. Such a great distance requires a relatively great number of stepping operations of a stepping motor for feeding print paper and a relatively great size of the printing head assembly and hence, of the entire printing apparatus. Additionally, tolerances in eccentricity of feed rollers for feeding print paper and in parallelism of a pair of platen rollers will have a significant influence on feeding of print paper so that it may be difficult to attain accurate adjustment of printing timings among the thermal heads. Consequently, a high quality of printing may not be attained.